Leading edge slats are commonly used and well understood. They are commonly driven on a circular track mechanism.
The simple circular track restricts design freedom when optimising both take-off and landing slat settings. Non-circular tracks are not generally used due to loads and kinematic complications. Applying an additional rotation to the slat beyond that provided by the track deployment enables greater design freedom. For example this allows a sealed setting at take-off where drag performance is important and a slotted setting at landing where CLmax is the design driver.
The track is commonly long and, when stowed, extends aft through the wing front structural spar. This requires a slat track “can” to be installed at each slat track to seal the fuel tank, reducing the fuel volume, and adding time and cost to the manufacture of the wing. Adding holes is especially inefficient for composite wing spars. Thus there is a strong driver to avoid front spar penetration in wing spars by the high lift mechanism. Once again applying an additional rotation to the slat enables a shorter track to be used whilst achieving a similar maximum slat deployment angle.
Double action slat mechanisms achieving an additional rotation about the slat heel exist. The most common form generates the additional rotation through the addition of a follower arm that is guided through a contoured rail as the slat is deployed by the track. An example is described in U.S. Pat. No. 3,272,458. A track is mounted within guide rollers, and driven by a rack and pinion mechanism. Rotation of the slat is effected by a push-pull rod driven by a bell crank which is carried by the track. One arm of the bell crank carries a cam follower which is entrapped within a camming track.